1. Field of the Invention
This invention relates generally to the field of mobile telephones, and more specifically pertains to pilot signal acquisition.
2. Description of Related Art
A Code Division Multiple Access (CDMA) digital communication system uses a special class of binary sequences with good correlation properties to identify and distinguish between the multiple mobile telephone units and base stations (BS). Pseudo noise (PN) binary sequences are used to synchronize the mobile unit to the base station signal. The pilot signal from each base station is uniquely identified by a different PN code offset.
The initial task of a mobile unit is to acquire the pilot signal transmitted by the base station for synchronization purposes. The pilot signal for a given base station is identified by a unique value of the PN code offset Tn, and the mobile telephone unit""s demodulator module is supplied with the value of this PN code offset. Only after successful pilot signal acquisition can the mobile unit register with the base station and initiate and receive calls. The PN code offset as perceived by the mobile unit is a function of the base station PN code offset Tn, fixed for that base station, and the mobile unit""s distance from the base station, called the signal path. Therefore, the PN code offset value is fixed at a given base station but will vary as perceived by the mobile unit as the mobile unit""s physical location and assigned base station changes with time.
The pilot signal acquisition is a major task and is performed by the searcher block hardware 10 of the Mobile Station Modem (MSM) chip 5. FIG. 1 is a simplified block-diagram of an MSM chip depicting only the pilot acquisition elements. The electromagnetic input signal received by the mobile unit is downconverted and input to the pilot searcher. The downconverted signal input to the pilot searcher is a combination of a) a data part D(n-Tn), b) a PN code part C(n-Tn); and c) a residual carrier part Cos(xcfx89e*n). The residual carrier part Cos(xcfx89e*n) is the result of frequency error in the mobile unit local oscillator. The frequency error is the local oscillator""s deviation from its intended normal frequency. It is desirable that the local oscillator frequency matches the base station frequency. The frequency error in the mobile unit local oscillator changes with temperature and age. The received pilot input signal is also offset by N (n), which is a Gaussian noise and interference function that makes the signal-to-noise ratio finite. The optimization of the N (n) offset requires utilization of some searching techniques to determine the optimized search parameters.
The searcher block 10 of the MSM chip 5 processes the incoming electromagnetic input signal and tries to determine the PN code offset Tn to be reported to the demodulator module 20. If the PN code offset Tn is determined properly, the mobile unit can successfully acquire control channels from the base station and establish forward and reverse links for synchronization purposes.
Typically, the pilot signal acquisition is performed using expensive temperature correcting hardware modules 30 to minimize frequency error. The hardware performs temperature compensation of some search parameters. The searcher block""s pilot signal acquisition task is non-dynamic with respect to most recent temperature values, and although it may use a fixed frequency estimation versus temperature table, it does not attempt to update current temperature and frequency values.
In another approach, the searcher block 10 uses a non-dynamic pilot signal acquisition task (i.e. xe2x80x9cprocessxe2x80x9d) comprising software, embedded in a microprocessor 40, also part of the MSM chip 5, with a searcher algorithm which chooses the particular search parameters and performs scheduling of the particular search sequence. The initial part of the pilot signal acquisition task consists of examining all possible PN code offsets Tn of the PN code part C(n) that possess good correlation properties. The search sequence is performed by the searcher software in the searcher block 10 hardware of the MSM chip. The MSM chip iteratively performs the multiplication, correlation, summation, and truncation operations of the searcher algorithm and reports the results to the embedded software in microprocessor 40. Certain search parameters are chosen by the searcher software and programmed into registers 50 of the MSM chip 5.
The searcher software is used in the mobile telephone to control the searcher block hardware 10 inside the MSM chip to quickly acquire and track base station pilot signals. The major considerations in designing the searcher algorithm are the speed of pilot signal acquisition and the success rate in acquiring a pilot signal of reasonable strength. Therefore, the search parameters are chosen to minimize the pilot signal acquisition time and maximize the probability of pilot signal acquisition.
The major obstacles in acquiring pilot signals are noise and interference that can be measured against the signal strength by the signal-to-noise ratio Ec/No (also referred to as pilot signal strength) and the parameter xcfx89e=2ΠFe. The parameter Fe is the residual frequency error, defined as the difference between the frequency of the base station pilot signal and the frequency of the mobile unit local oscillator. Each mobile unit is residual frequency error Fe is a function of temperature, age, voltage stability and the type of crystal used to build the local oscillator circuit.
The MSM chip hardware registers 50 are used to hold parameters programmable by the searcher software, such as Searcher_Gain register for G parameter, Search_Integrate_Time register for the integration length parameter Nc, Search_Acc_Passes register for the number of integration passes (Nn parameter), and Search_Num register for the sweep window size (Ws parameter). These four major parameters are the building blocks of each search stage, and different stages are the building blocks of the searcher algorithm. A stage is a particular search conducted for a pilot signal, using a given set of values for these major parameters.
The Nc parameter describes the length of coherent energy integration. Coherent integration suffers significant degradation when the residual frequency error Fe reaches a predetermined large value and the integration has to be stopped. The estimate of the effect of a frequency error on the coherent energy degradation, which is a correlation loss as a function of the residual frequency error Fe, may be described by the following equation for frequency degradation coefficient L:
L(Nc, Fe)=[sin (Nc*Fe)/Nc*Fe]2
Since the L function is a decreasing function dependent on Nc and Fe parameters, the presence of a larger value of the residual frequency error Fe allows use of a smaller coherent energy integration length Nc. At a certain residual frequency error Fe value the searcher software determines that the correlation value is degraded enough to significantly reduce the probability of pilot signal acquisition.
Ideally, the mobile unit should have a short acquisition time and good searcher sensitivity, which will be obtained if a low value pilot signal strength can be acquired by the mobile unit. However, there is usually a tradeoff between the two values. The total acquisition time the searcher spends searching all hypotheses is roughly on the order of Nc*Nn, where Nn is the number of integration passes of integration length Nc.
If the residual frequency error Fe is present and unknown to the searcher, the acquisition time is significantly higher than if it is known. To provide sufficient total correlation length, several (Nn) passes of integration length Nc should be made, and the energy accumulated in each coherent pass should be squared and added together to produce the total energy estimate. Since this non-coherent addition is not as effective as coherent integration against the noise and interference, reducing the Nc number by half will require more than doubling the Nn value to acquire a pilot signal of the same strength Ec/No. Thus, to shorten the long acquisition time for the unknown value of the residual frequency error Fe, the number of integration passes has to be increased to account for reduced integration length Nc.
In yet another software approach the searcher pilot signal acquisition task uses a static frequency error estimation table and there is no attempt to track recent temperature and frequency error values. There, the temperature compensation of the residual frequency error Fe is performed by a searcher task, which makes decisions about the search strategy based on the state of a hardware register state and the next hardware task is determined by the static frequency estimates.
Therefore, there is a need for an efficient architecture and method for dynamic temperature compensation and search parameter selection in order to reduce the time required for pilot signal acquisition.
The preceding and other shortcomings of prior systems are addressed and overcome by various embodiments of the present invention. The present invention is an improved method and apparatus for dynamic temperature compensation and stage selection within a pilot signal acquisition circuit.
Since it is known that frequency error Fe is a function of temperature, a dynamic Temperature Compensation Table (TCT) is maintained. The values in the TCT correspond to frequency error estimates of the local oscillator at particular temperatures. After a pilot signal is acquired this table is updated every 30 seconds. The TCT is stored in non-volatile memory. In addition to the TCT a smaller Temperature Array (TA) stored in volatile memory is used to record the most recent temperatures and frequency errors. The TA is updated at the same time the TCT is updated. However, since the TA is stored in volatile memory its contents are erased on power down and reset to zero each time the phone is powered up. Therefore, the TA holds the most recent frequency error and temperature estimates from the current phone on cycle. Since the TA holds no valuable information upon phone power up the phone must use the pilot searcher stage that is able to acquire a pilot signal without any frequency error estimate. Once the pilot is acquired a first time, updated values will be written to both the TCT and the TA. The software sets a status flag to indicate that values have been written into the TA.
If the pilot signal is subsequently lost and the phone is required to reacquire a pilot signal the acquisition software sees that the status flag has been set and that temperature and frequency error values exist in the TA. If the exact current temperature is saved in the TA the corresponding frequency error is known and a fast pilot searcher stage can be used. If the current temperature is within a predetermined range of the values saved in the TA, e.g. within two units, a weighted frequency error estimate is calculated based on the values stored in the TA and the calculated value of the frequency error at the current temperature is stored in the TCT. The software then is able to direct the phone to use a particular pilot searcher stage depending upon the results.
The foregoing and additional features and advantages of the present invention will become further apparent from the following detailed description and accompanying drawing figures that follow. In the figures and written description, numerals indicate the various features of the invention, like numerals referring to like features, throughout the drawing figures and the written description.